The present invention relates to rotary torque-transmitting coupling devices, and more particularly to such devices wherein heat is generated as a result of the torque transmission, and the ability to dissipate such heat represents a limiting factor on the torque-transmitting capability of the device.
Although the present invention may be used with various types and configurations of rotary torque-transmitting coupling devices, it is especially adapted for use with rotary fluid coupling devices, and will be described in connection therewith.
Rotary fluid coupling devices of the type which may benefit from the use of the present invention have found many uses, one of the most common of which is to drive the cooling fan associated with the radiator of vehicle engines. Such coupling devices are frequently referred to as "viscous fan drives" because such couplings utilize a high-viscosity fluid to transmit torque, by means of viscous shear drag, from an input coupling member (clutch) to an output coupling member (housing) to which is bolted the cooling fan.
More specifically, this invention is especially advantageous when used on a relatively high torque viscous fan drive, i.e., a fan drive which is capable of transmitting to the cooling fan in the range of about 2 horsepower to about 12 horsepower. Typically, such high torque or high horsepower fan drives include an output coupling assembly comprising a cast aluminum housing and a die cast aluminum cover. The input coupling member and the die cast cover normally define a plurality of interdigitated lands and grooves which define the shear space. When this shear space is filled with viscous fluid, torque is transmitted from the input coupling member to the output coupling assembly, in response to rotation of the input coupling.
During torque transmission, substantial heat is generated as a result of the shearing of the viscous fluid between the interdigitated lands and grooves. The amount of heat generated is generally proportional to the "slip" speed of the fan drive, i.e., the difference between the speed of the input coupling and the speed of the output coupling. It is generally well understood by those skilled in the art that, for many rotary torque-transmitting coupling devices, and especially for viscous fan drives, the ability to transmit torque is limited by the ability of the device to dissipate the heat generated as a result of torque transmission. For example, in a viscous fan drive, if the temperature of the viscous fluid exceeds a certain maximum temperature, the result will be a deterioration in the viscous properties of the viscous fluid, resulting in a loss of torque-transmitting capability of the fluid.
In viscous fan drives of the type described above in which a cast cover forms part of the shear space, the cast cover is also the primary heat dissipating element of the device. Therefore, it has been conventional practice in the viscous fan drive art to have a plurality of cooling fins cast integrally with the cover. It is now becoming recognized by those skilled in the art that the majority of the heat is dissipated by fins which are located axially adjacent the viscous shear area of the coupling device. It has also been generally accepted by those skilled in the art that heat dissipation is generally proportional to the total length of cooling fins located in the primary heat dissipation region.
As may be seen by reference to U.S. Pat. Nos. 2,963,135 and 3,075,691, it has been conventional practice since the very early days of the viscous fan drive industry to have the cooling fins oriented radially, primarily to facilitate the radially outward flow of air which carries away the heat transmitted from the shear space to the cooling fins. As may be seen from the above-referenced patents, it also has been conventional practice to alternate longer and shorter radial fins in an attempt to increase the total length of cooling fins, without having the fins disposed too close together. As may be seen by reference to U.S. Pat. No. 4,134,484, it is still common practice, after all these years, to utilize the same basic cooling fin pattern.
As the torque-transmission requirements of viscous fan drives have increased over the years, thus increasing the need to dissipate heat, the primary solution attempted by those skilled in the art has been to increase the total number of cooling fins, resulting in adjacent fins being closer together. This is a generally undesirable solution because the die required to cast such a cover becomes much more expensive, requires more maintenance, and has a shorter die life. Therefore, it is generally recognized that it is undesirable to have adjacent cooling fins closer together than some predetermined, minimum distance of separation.
Given the need for greater heat dissipation, and the recognized limitations on increasing the number of cooling fins as discussed above, those skilled in the art have even attempted to add additional structure to increase the flow of air over the cooling fins. U.S. Pat. No. 4,181,205 discloses a viscous fan drive including an arrangement of fan blades located adjacent the front of the fan drive cover, in an attempt to increase the amount of heat dissipated from the cooling fins by increasing the flow of air over the cooling fins. Such an arrangement, although possibly effective, would add substantially to the size, weight, complexity, and cost of the fan drive and would in addition usurp a portion of the fan drive output horsepower, simply to drive the fan blade arrangement.
Accordingly, it is an object of the present invention to provide an improved rotary torque-transmitting coupling device of the type having cooling fins which are generally radially oriented to facilitate radial airflow, wherein the total fin length is increased to increase heat dissipation.
It is another object of the present invention to provide a coupling device which is capable of achieving the above-stated objects and wherein the element equipped with the cooling fins is a die casting, made in accordance with good die casting practices.
While those skilled in the art have been attempting to provide viscous fan drives having greater heat dissipation capability, there has, at the same time, been a demand by vehicle manufacturers for engine components such as viscous fan drives to be lighter and less expensive. Thus, it is not entirely satisfactory to achieve greater heat dissipation capability in a viscous fan drive simply by increasing the total fin length (and fin area), which also increases the total material content and material cost.
Although various cooling fin configurations have been illustrated in various prior art patents, it has been conventional practice in designing relatively high-torque viscous fan drives to provide cooling fins which have a generally uniform nominal height over most of the radial extent thereof.
Accordingly, it is another object of the present invention to provide a coupling device which is capable of achieving the previously-stated objects, but wherein the cooling fins are configured to limit the fin area, and thus the material content and cost, without any substantial reduction in heat dissipation capability.
It may be seen from the above objects that it is an overall object of the invention to provide a coupling device having greater cooling efficiency, i.e., a greater heat dissipation capability per unit volume of material.